Stone Crib

A stone crib is a designed basin that collects the first ½ inch of stormwater from a site and reduces its velocity by passing it through the basin. This action promotes sedimentation and reduces the temperature of runoff by utilizing the heat exchange capacity of the stone. As stormwater enters the stone crib, it is passed through a layer of pervious pavement block and pea gravel, which filter out sediment and other particles. It then mixes with the stone and any stored water in the crib, exchanging heat with the cooler water. Trash and other large debris are collected upstream of the device by a Gabion. The cooled water is released slowly to a stable outlet.

Stone cribs are widely applicable, particularly in temperature sensitive watersheds. As these devices are only designed to provide treatment for the first ½ inch of runoff, large storm events may overwhelm the capacity of the practice. As a result, they are best used in conjunction with other management practices.

Advantages

• Reduces the temperature of stormwater
• Widely applicable, especially in temperature sensitive watersheds
• Requires minimal land area
• Relatively cost-effective
• Low maintenance

Disadvantages

• Limited effectiveness with large storm events

The basin should have side slopes of 3:1 or flatter, which should be lined with a layer of non-woven, highly permeable geotextile filter fabric. The filter fabric prevents the soil from mixing with the stone and thus reducing the capacity of the basin. The bottom of the basin, however, should not be lined as it may reduce the infiltration ability of the practice.

A 3-4 foot layer of stone fills the majority of the basin and is placed on top of the geotextile filter fabric. The stone should be sized based upon the individual hydrologic conditions of the site, but should be well graded, with ½ of the stone larger than the median stone size, and free of fines and sand.

The stone is covered with a layer of pea gravel that is at least 1 foot thick. The pea gravel should be properly compacted to allow maintenance vehicles access to the stone crib without damaging the structure. To prevent the pea gravel from settling and filling the pore spaces in the stone, a layer of non-woven, highly permeable geotextile filter fabric should be placed between the stone and the pea gravel. Finally, a layer of pervious pavement blocks is set on top of the pea gravel. The pavement blocks should be at least 6 inches thick and at least 30% pervious to allow water to infiltrate the basin.

The stone crib should be sized depending upon the individual characteristics of the site, but should be large enough to handle the first ½ inch of runoff from the site. Generally, the basin will have an area of 2,000-4,000 square feet with a depth of 3-4 feet.

Stormwater should be conveyed into the stone crib through a stone-lined channel and a Gabion. These structures reduce the flow velocity of the water and trap sediment and other debris before they reach the stone crib. They increase the longevity of the practice, as larger particles quickly clog the pores of the structure and reduce the effectiveness of the practice. The gabion should be at least 3 feet square with at least 1 foot extending above the surface. All other design considerations for inlets should follow the criteria set forth in the Lined Waterway or Outlet and Gabion sections of this appendix.

The stone crib should discharge at non-erosive velocities to a stable outlet.

• Slopes surrounding the practice should be 6:1 or flatter to allow access for maintenance vehicles
• Stormwater should be delivered to the stone crib at non-erosive velocities

• Stone cribs should be inspected after large storm events – any necessary repairs should be made immediately
• Accumulated sediment, leaves, trash, and other debris should be removed as needed

Stone cribs are designed to reduce the temperature of the first ½ inch of runoff from a site. A properly sized practice can be used to provide thermal mitigation for sites in thermally sensitive areas.